1. Technical Field to Which the Invention Pertains
The present invention relates to glass-ceramics, and to a substrate of a reflecting mirror (hereinafter referred to as xe2x80x9creflecting mirror substratexe2x80x9d), the substrate being formed from the glass-ceramic.
2. Background Art
Glass-ceramics, exhibiting excellent heat resistance, are useful materials for forming various apparatus parts used at high temperatures. An example of an apparatus part used at high temperatures includes a reflecting mirror. Together with a light source lamp mounted thereon, the reflecting mirror constitutes an illumination apparatus. Since the temperature of the reflecting mirror rises by heat generated from the light source lamp, the reflecting mirror is required to have high heat resistance. Therefore, the substrate of the reflecting mirror must also exhibit high heat resistance. Conventional reflecting mirror substrates are generally formed from PYREX (registered trademark) glass exhibiting excellent heat resistance.
In recent years, light source lamps of high luminance have been employed in illumination apparatuses and cineprojectors. In accordance with this trend, a reflecting mirror which constitutes an illumination apparatus employing a light source lamp of high luminance is required to have enhanced heat resistance.
In order to meet such requirements, Japanese Patent Publication (kokoku) No. 7-92527 discloses a reflecting mirror substrate formed from glass-ceramic containing, as primary components, a xcex2-spodumene solid solution and a xcex2-eucryptite solid solution. Since the glass-ceramic, which is known as a low-expansion glass-ceramic, exhibits excellent heat resistance, a reflecting mirror of high heat resistance can be formed from the glass-ceramic.
U.S. Pat. No. 5,786,286 discloses a reflecting mirror substrate formed from another type of glass-ceramic. Unlike the aforementioned reflecting mirror substrate, the substrate disclosed in this patent is not used at high temperatures. The substrate is formed from a glass-ceramic containing hexacelsian as a primary crystal phase and having a degree of crystallinity of about 50%, and has a thermal expansion coefficient xcex1 (xc3x9710xe2x88x927/xc2x0C. as high as 78 to 88.
In the course of forming the aforementioned substrates, the glass raw materials of the glass-ceramic must be melted at a temperature as high as 1,500xc2x0 C. or more, since the glass raw materials have high melting points. The substrates are formed by forming methods, such as press forming, roll forming, casting, or blow forming. In such forming methods, since the glass raw materials are melted at 1,500xc2x0 C. or higher prior to formation of a substrate, the surface of the forming die employed is oxidized, and the surface is roughened at an early stage, resulting in a shortened service life for the forming die.
Particularly, when the surface of a forming die is roughened, the resultant substrate fails to have the desired surface roughness. When the surface of the forming die is roughened, in order to impart the predetermined surface roughness to a substrate to be formed, the forming die must be exchanged for a new forming die at an early stage, resulting in a great increase in the costs required to prepare a large number of forming dies to be exchanged and the costs required for exchanging the forming dies.
In previously filed Japanese Patent Application No. 2000-299507, the present applicant proposed a reflecting mirror exhibiting excellent heat resistance, in which the substrate of the mirror is formed from glass-ceramic containing primary components differing from those of glass-ceramic containing a xcex2-spodumene solid solution and a xcex2-eucryptite solid solution, and exhibiting heat resistance similar to that of the glass-ceramic.
The reflecting mirror includes a glass-ceramic substrate and a thin reflection film deposited on the substrate. The substrate is formed from a glass-ceramic having celsian as a primary crystal phase and containing SiO2, Al2O3, and BaO as primary components and TiO2 as a crystal nucleus component, and has a thermal expansion coefficient xcex1 (xc3x9710xe2x88x927/xc2x0C. of 30 to 45. The reflecting mirror does not involve the aforementioned problems attributed to the formation of reflecting mirror substrates, and exhibits excellent heat resistance.
The present inventors have conducted further studies on glass-ceramics having celsian as a primary crystal phase and containing SiO2, Al2O3, BaO, and TiO2 as primary components, the glass-ceramic constituting the aforementioned reflecting mirror substrate, and have found that Li2O serving as a specific modification component greatly contributes to crystal formation in the glass-ceramic, and that a glass-ceramic containing no hexacelsian crystals or very small amounts of hexacelsian crystals; i.e., glass-ceramic whose crystal phase is substantially formed of celsian alone, can be obtained by varying the Li2O content of the glass-ceramic.
The present invention has been accomplished on the basis of this finding. An object of the present invention is to provide a very useful glass-ceramic material for forming apparatus parts exhibiting excellent heat resistance, mechanical strength, and optical properties; and a substrate which is formed from the glass-ceramic material and constitutes a high quality reflecting mirror.
The present invention relates to a glass-ceramic, and a first glass-ceramic according to the present invention has a celsian primary crystal phase comprising SiO2, Al2 O3, BaO, and TiO2 as primary components and Li2O as a modification component, and is characterized in that the amount of Li2O falls within a range of 0.05 to 1.0 wt. %.
A second glass-ceramic according to the present invention has a celsian primary crystal phase comprising SiO2, Al2O3, BaO, and TiO2 as primary components and Li2O and K2O as modification components, and is characterized in that the amount of Li2O falls within a range of 0.05 to 1.0 wt. %, and the amount of K2O is 1.5 wt. % or less.
Each of the glass-ceramics of the present invention may further comprise one or more modification components selected from the group consisting of Na2O, P2O5, B2O3, Sb2O3, ZnO, and Bi2O3.
The present invention also relates to a reflecting mirror substrate. The reflecting mirror substrate of the present invention is a substrate on which a thin reflection film is deposited to thereby form a reflecting mirror, the substrate being formed from the first or second glass-ceramics of the present invention.
In the reflecting mirror substrate of the present invention, preferably, the glass-ceramic constituting the substrate has a thermal expansion coefficient xcex1 (xc3x9710xe2x88x927/xc2x0C.) in a range of 30 to 45, and the glass-ceramic has a crystal grain size in a range of 0.1 to 1 xcexcm.
The reflecting mirror substrate of the present invention has a flexural strength in a range of 125 to 155 Mpa at room temperature, in a range of 145 to 175 Mpa at 300xc2x0 C., and in a range of 180 to 220 Mpa at 600xc2x0 C., and an elastic modulus in a range of 80 to 85 Gpa at room temperature, in a range of 70 to 75 Gpa at 300xc2x0 C., and in a range of 35 to 40 Gpa at 600xc2x0 C.
In the reflecting mirror substrate of the present invention, preferably, the shortest wavelength of light which passes through the glass-ceramic constituting the substrate is at least 800 nm when the glass-ceramic has a thickness of 0.1 mm, and the wavelength of light which passes, at a transmittance of 50%, through the glass-ceramic constituting the substrate is at least 850 nm when the glass-ceramic has a thickness of 0.1 mm.
In the reflecting mirror substrate of the present invention exhibiting the aforementioned properties, which has a thickness of 3 to 6 mm, the shortest wavelength of light which passes through the glass-ceramic constituting the substrate is at least 850 nm, and preferably, the shortest wavelength of light which passes through the glass-ceramic constituting the substrate is at least 1,000 nm.
In the first glass-ceramic of the present invention, a small amount of Li2O, serving as an essential modification component, enhances meltability of the glass, promotes precipitation of celsian crystals, and increases the rate of growth of the celsian crystals. Therefore, by virtue of the synergistic effect of the primary components and the modification component, the melting point of the glass raw material of the glass-ceramic decreases to 1,450xc2x0 C. or lower, which is considerably lower than the melting point of the glass raw material of the aforementioned conventional glass-ceramic; i.e., 1,500xc2x0 C. or higher. In addition, the first glass-ceramic is substantially formed of a celsian crystal phase alone, by virtue of a crystallization temperature as low as at least 800xc2x0 C. In the second glass-ceramic of the present invention, a small amount of K2O, which is added as a modification component together with Li2O, aids the function of Li2O.
Therefore, the glass-ceramic of the present invention is a very useful glass material for forming various apparatus parts exhibiting excellent heat resistance, mechanical strength, and optical properties, and is a suitable material for forming such apparatus parts. The glass-ceramic is a suitable material for forming, for example, a reflecting mirror substrate, which is generally formed from a glass material.
When the glass-ceramic is formed into an apparatus part, the melting temperature of the glass raw material of the glass-ceramic can be determined to be 1,450xc2x0 C. or lower, which is considerably lower than that of a conventional glass raw material (i.e., 1,500xc2x0 C. or higher). Therefore, in the course of forming the apparatus part, oxidation of the surface of the forming die to be employed, which is attributed to heating at a high temperature, can be prevented, and roughening of the surface of the die at an early stage can be prevented, resulting in an extended service life for the forming die. As a result of the extended service life of the forming die, the frequency of exchanging forming dies can be reduced, and the number of forming dies to be prepared for exchange can be greatly reduced, contributing to a great reduction in the costs required for preparing forming dies and the costs required for operation of exchange.
Furthermore, since roughening of the surface of the forming die can be prevented, the following advantages are obtained: the roughness of the surface of the forming die, which is determined in accordance with the surface roughness as desired of an apparatus part to be formed, can be maintained for a long period of time; an apparatus part having a desired surface roughness can be easily formed; and apparatus parts having an appropriate surface roughness can be reliably formed over a long period of time.
The glass-ceramic of the present invention is a suitable material for forming a substrate on which a thin reflection film is deposited to thereby form a reflecting mirror (i.e., a reflecting mirror substrate). The thermal expansion coefficient xcex1 (xc3x9710xe2x88x927/xc2x0C.) of the glass-ceramic for the reflecting mirror substrate can be determined to be in a range of 30 to 45, and the crystal grain size of the glass-ceramic can be determined to be in a range of 0.1 to 1 xcexcm. Therefore, the reflecting mirror substrate exhibits further excellent mechanical properties, thermal properties, and optical properties. A reflecting mirror including the substrate is suitably employed as a high-functional light source lamp of high luminance which generates a considerable amount of heat, and the reflecting mirror can be used over a long period of time.
In the present invention, when the compositions of glass raw materials are varied and crystallization temperature, etc. are regulated, various glass-ceramics of different properties, such as different flexural strengths, elastic moduli, and light transmittances, can be formed. By selecting an outstandingly excellent glass-ceramic from among such glass-ceramics of different properties, a high-quality reflecting mirror substrate exhibiting the below-described properties can be provided.
That is, a reflecting mirror substrate of high durability exhibiting excellent mechanical properties and thermal properties can be formed, which has a flexural strength in a range of 125 to 155 Mpa at room temperature, in a range of 145 to 175 Mpa at 300xc2x0 C., and in a range of 180 to 220 Mpa at 600xc2x0 C., and an elastic modulus in a range of 80 to 85 Gpa at room temperature, in a range of 70 to 75 Gpa at 300xc2x0 C., and in a range of 35 to 40 Gpa at 600xc2x0 C.
Also, a reflecting mirror substrate exhibiting excellent optical properties can be formed, in which the shortest wavelength of light which passes through a glass-ceramic constituting the substrate is at least 800 nm when the glass-ceramic has a thickness of 0.1 mm, and the wavelength of light which passes, at a transmittance of 50%, through the glass-ceramic constituting the substrate is at least 850 nm when the glass-ceramic has a thickness of 0.1 mm.
Also, a reflecting mirror substrate can be formed that exhibits all the aforementioned properties; i.e., a reflecting mirror substrate exhibiting excellent mechanical properties, thermal properties, and optical properties, which has a flexural strength in a range of 125 to 155 Mpa at room temperature, in a range of 145 to 175 Mpa at 300xc2x0 C., and in a range of 180 to 220 Mpa at 600xc2x0 C., and an elastic modulus in a range of 80 to 85 Gpa at room temperature, in a range of 70 to 75 Gpa at 300xc2x0 C., and in a range of 35 to 40 Gpa at 600xc2x0 C., in which the shortest wavelength of light which passes through a glass-ceramic constituting the substrate is at least 800 nm when the glass-ceramic has a thickness of 0.1 mm, and the wavelength of light which passes, at a transmittance of 50%, through the glass-ceramic constituting the substrate is at least 850 nm when the glass-ceramic has a thickness of 0.1 mm.
In view of the features of the aforementioned reflecting mirror substrates, the most preferable reflecting mirror substrate exhibits all the aforementioned properties and has a thickness of 3 to 6 mm, in which the shortest wavelength of light which passes through a glass-ceramic constituting the substrate is at least 850 nm, preferably at least 1,000 nm. The reflecting mirror substrate exhibits further excellent mechanical properties, thermal properties, and optical properties.